A cocktail invented around 1918 in either London or Paris. To make it, you take eight parts brandy or cognac, mix with two parts orange liqueur (Cointreau, Grand Marnier, or another triple sec), and one part lemon juice. To serve, you mix the ingredients in a shaker half-full with ice, then strain and serve in a sugar-rimmed glass, and finally garnish with a strip of lemon rind. You can also use vodka, bourbon, or gin in place of the brandy and/or cognac

Man, the bartender at that restaurant made the best damn sidecar I ever had.

When you are out with a friend and a group of his friends, who you aren't well acquainted with. This put's you in the sidecar,(as in motorcycle sidecar) your friend is driving and you are awkwardly along for the ride. with nothing to control or keep you occupied... (beside your nerdy cell phone). And the subject of the group conversation turns to something you have no clue about, (Inside jokes, Old school/work stories) It's hard to look cool in the sidecar.

Man! I went to a party with Mike last night, there was a bunch of his old highschool buddies there.
Mike and his buddies started talking about their favorite rugby teams, I have no clue about rugby. I got put in the sidecar!

Anyone who is in a relationship/partnership who is clearly not wearing the metaphorical trousers, or in more extreme cases, only worked with/dated by the other for superficial reasons I.E - money or looks, and while everyone else can see this, they cannot.

Good looking woman walks into a crowded bar and pushes her way to the front, and without asking the small nerdy guy on HER arm what he wants, she orders the drinks.

ONLOOKER 1: "woah check out that blonde!"
ONLOOKER 2: "...check out the sidecar"